The present invention is concerned with a process for the recovery of chlorine from metal chlorides. More specifically, the invention relates to a process for dechlorinating iron chloride mixed with several other metal chlorides to produce chlorine gas. The process has the ability to recover chlorine from these chlorides in the presence of carbon and other materials such as oxides which are present in typical wastes generated during the chlorination of synthetic rutile.
Titanium tetrachloride is conventionally produced from either ilmenite or rutile (including synthetic rutile) in a fluidised bed at around 1000xc2x0 C. Chlorine, a carbon rich material such as petroleum coke and a titanium-bearing material are fed into the fluidised bed chlorinator and titanium tetrachloride vapour leaves the system in the gas phase. It is subsequently condensed, purified and used in the production of either pigment or titanium metal. Chlorine is recycled to the chlorinator.
Impurities such as Mg, Mn, Al in the feed are chlorinated to varying degrees. Iron in the feed chlorinates readily and is sufficiently volatile to leave the reactor with the effluent gas. The chlorides of these metals are condensed from the gas phase at temperatures around 200xc2x0 C., while titanium tetrachloride, which has a lower boiling point, remains as a vapour. Non titanium metal chlorides are therefore amenable to separation by differential condensation. They are recovered as solids in the chlorinator off gas cooling system.
The condensed stream from the chlorinator typically contains chlorides such as FeCl2, MnCl2, MgCl2 and AlOCl, as well as large quantities of coke and synthetic rutile which are blown over from the chlorinator. This material is referred to as the chlorinator waste.
The chlorinator waste is subsequently disposed of by whatever means are most acceptable from an environmental point of view. Techniques include direct dumping in mineshafts, blending into concrete for low-strength marine applications and wet chemical treatment to produce iron oxide and HCl (or NaCl with usage of caustic soda). In some pigment plants the waste is currently disposed of by mixing it with CaO and water which react with the chlorides to form oxides and CaCl2. The oxides and other solids are thickened, de-watered and returned to the mine. The CaCl2 liquor is discharged into the ocean.
These forms of disposal mean that valuable chlorine present as chlorides in the chlorinator waste is not recovered. These options are also undesirable because they are either environmentally sensitive or costly in terms of consumables and generally call for minimisation of iron chloride production. For this reason, rutile is the feedstock of choice for chlorination. It contains less iron than ilmenite. In commercial processes, therefore, ilmenite is first converted to synthetic Futile by removing substantially all of the iron by appropriate pre-treatment processes and the synthetic rutile is subsequently used in the chlorination process.
An alternative to simply disposing of the chlorinator waste is to react the waste directly with oxygen to form oxides and recover the chlorine as shown schematically in FIG. 1. The oxides generated could be useful as landfill or smelter feed, or returned to the mine and the chlorine recycled to the chlorinator. Such a dechlorination process has the potential to reduce the cost of pigment production by reducing the quantity of fresh chlorine required, and by significantly reducing the consumption of water.
The chlorination industry has a long-standing need for a process which can convert iron chloride into chlorine and iron oxide. In the production of titanium tetrachloride the chlorine could be directly recycled to the chlorinator, thereby decreasing the need for chlorine-make-up. For such a process to be effective, the iron oxide produced would need to be sufficiently free of chlorine to allow disposal as landfill or smelter feed with little or no further treatment. The availability of such a process would create the potential for processes such as:
Direct chlorination of ilmenite in the case of xe2x80x9ccleanxe2x80x9d (low Ca, Mg, Mn, Al) deposits. This simultaneously obviates the need for synthetic rutile production and offers a solution to the iron chloride disposal problem.
Ilmenite conversion to synthetic rutile by partial chlorination with subsequent alkali removal in the case of dirty (high Ca, Mg and/or Mn) deposits. This would present an alternative to more conventional methods of impurity removal by reduction and leaching and would lead to reduced waste disposal requirements.
A general waste disposal route for undesired iron chloride and other chlorides which yields chlorine for direct recycle to chlorination systems. This includes the use of skid mounted plants to process accumulated iron chloride waste at existing chlorinator sites, particularly in Europe and USA.
The proposed technologies for chlorine recovery, as disclosed in the literature, are not sufficiently selective and/or have unattractive scale-up features.
The Du Pont recirculating fluidised bed approach (U.S. Pat. Nos. 3,793,444, 4,144,316 and 4,174,381) claims greater than 95% chlorine removal, leaving a nominal 3-5% leachable chloride in the solid phase and rendering it unsuitable for direct disposal.
The Mitsubishi vapour-phase approach (S Fukushima and Y. Sugawara, Light metals, AIME 1974), claims 90% chlorine removal and in addition, has perceived scale-up limitations.
The Mineral Process approach (U.S. Pat. No. 4,140,746) comprises partial dechlorination of ferric chloride to ferrous chloride in the presence of a reducing agent such as sulfur or chlorine polysulfides to produce a chloride compound in the first step. In the second step the ferrous chloride is oxidised to ferric chloride and ferric oxide and the ferric chloride is recycled to the first step. The chlorine values were recovered as compounds containing chlorine but not as chlorine gas.
SCM Chemical""s U.S. Pat. No. 4,624,843 (inventor M Robinson, 25 Nov. 1986) refers to control of the proportions of the iron chloride and carbon in the blown over material from the chlorinator by controlling the iron oxide concentration in the feed to the chlorinator. The iron oxide is claimed to be introduced through the addition of ilmenite and/or rutile slag. The chlorinator is therefore being used as a selective chlorinator or beneficiator to upgrade other Ti bearing materials. The blown over carbon is controlled between 7.5 and 20% carbon based on the carbon plus iron chloride only, the blow over of rutile is not specifically mentioned. It is claimed that the iron chloride is formed as ferric chloride under the conditions used in the selective chlorinator.
The dechlorinator claimed in U.S. Pat. No. 4,624,843 is based on the introduction of more than one oxygen stream into the reactor. A first stream introduced at the base of the fluidised bed is intended to react with the carbon content of the feed and maintain the bed temperature at between 500 and 1050xc2x0 C., xe2x80x9cpreferably at least at 600xc2x0 C.xe2x80x9d. This step seems to be intended to vaporise the ferric chloride although the patent is not very clear on this aspect. A second or more oxygen streams, which appear to need preheating, are introduced above the bed to react with the ferric chloride vapour.
With the dechlorination reaction occurring in the gas phase above the bed between ferric chloride vapour and oxygen, the conversions are unlikely to be complete because of residence time constraints. This implies that iron chloride recycle levels will be significant. Moreover, the iron oxide will be produced as fine dust which increase the probability of accretion formation at the bed exit.
We have discovered that waste obtained from typical chlorinators a) the iron is present as ferrous chloride, b) the conversion of ferrous to ferric chloride is reasonably rapid during dechlorination and c) the conversion of chloride to oxide is helped by using a fluidised bed of particles. The need for the type of pseudo multi-stage operation claimed in U.S. Pat. No. 4,624,843 makes reactor operation and control more difficult with no perceived gain.
SCM Chemicals"" U.S. Pat. No. 4,994,255 (inventor C K Hahn, 19 Feb. 1991) is a process patent which claims several hypothetical process schemes in which a major feature is the claim to apparent complete or near complete separation of carbon from the rest of the blown over material. It is claimed that xe2x80x9cthe carbon and ore solids can be most efficiently removed by using a ceramic filter bag in a baghouse. The temperature can be as high as 800xc2x0 C.xe2x80x9d However, the patent goes on to state that the xe2x80x9ccarbon content in the condensed FeCl2 under these conditions is below about 12% and usually, the removal of carbon and ore is almost completexe2x80x9d. The patent also refers to U.S. Pat. No. 4,094,954 and states that xe2x80x9cthe carbon, together with any ore in the offgas stream can be separated by conventional means such as using a hot cyclone separator, electrostatic separator or a knock-out pot. Their claim implies, erroneously, that at 800xc2x0 C. the ferrous chloride is in vapour form.
The conceptual flowsheets in FIGS. 1, 2, 4 and 5 in U.S. Pat. No. 4,994,255 are not substantiated by any results. It is clear from the descriptions and diagrams that the patent assumes that substantially pure FeCl2 is treated in the FeCl2 oxidiser. The FeCl2 oxidiser appears to be based on an inert material which is partly removed with the iron oxide coating. Because their patent is based on pure or substantially pure iron chloride, an inert bed is necessary to avoid potential problems due to molten chlorides.
U.S. Pat. No. 4,994,255 does not reveal the optimum operating conditions such as temperature and oxygen/waste feed ratio for maximum chlorine recovery at economic rates of chlorine production other than to say that the temperature should be below the melting point of FeCl2 (674xc2x0 C.).
We have discovered that in a real chlorinator waste the conversion rates of the chlorides can be quite rapid despite the presence of liquid phases which melt at much lower temperatures when other metal chlorides such as those of Mn and Mg are present along with iron chloride. Our invention is not limited to pure iron chloride.
U.S. Pat. No. 4,144,316 for ferric chloride conversion claims the need to add a catalyst (sodium chloride) to achieve satisfactory conversion in a fluidised bed of ferric chloride particles. U.S. Pat. No. 3,944,647, restricted to the treatment of ferric chloride, also claims the use of sodium chloride to produce a liquid sodium ferric chloride salt complex.
Various publications have dealt with the problems of obtaining high conversions with ferric chloride. A paper by Olsen (Olsen R S, in Recycle and Secondary recovery of metals, 1985, Taylor P R et al editors, pp771-783) gives results of a preliminary study of the treatment of ferrous chloride in a fluidised bed but the reported conversions were low. We have discovered that in a real chlorinator waste, the conversion rate and final conversion of the chlorides in the waste are high without the need for any catalyst addition.
U.S. Pat. No. 3,642,441 (Falconbridge Nickel Mines) appears to be based on a complex process with a multiplicity of streams and relies on the endothermic reaction of xe2x80x9cexplosivexe2x80x9d mixtures of metal chlorides with water and combustion of the mixtures. Significant quantities of hydrogen bearing gases will result and even if the process were capable of being scaled up satisfactorily, it is likely to be more suited to HCl rather than chlorine production.
U.S. Pat. No. 3,325,252, and related U.S. Pat. No. 3,050,365, are based on a two-zone furnace using molten iron chloride feed. Based on the quoted example, liquid ferric chloride is fed under pressure through a spray head to the inner tube of a burner with oxygen supply through an outer tube. It is claimed that the reaction is completed in a second, lower zone, into which alumina particles are fed. This zone was maintained at about 450xc2x0 C. but the rate of chlorine recovery would have been low under these conditions. The process seems to be restricted to iron chloride levels less than 10% in the feed. Higher ferrous chloride levels are unlikely to give satisfactory chlorine recoveries because of short residence times and the absence of inert substrate particles in the upper zone and the slow kinetics in the lower zone. Accretion formation is also seen as a major problem. U.S. Pat. No. 3,092,456 attempts to address this issue but a practical solution is not simple.
The vapour phase contacting method claimed by Mitsubishi (U.S. Pat. No. 4,073,874 to Fukushima et al) is based on treating ferric chloride. The ferric chloride vapour and several streams of oxygen, injected through nozzles, react with each other in an enclosure. It is claimed that in initial work there were significant operational problems caused by the formation of iron oxide accretions at the point where the chloride vapour stream impinged on the oxygen stream. It appears that the problem (Fukushima S and Sugawara Y in Light Metals Technology II, 1974, pp443-466) with accretions was not completely eliminated.
In summary, the application of fluidised bed or other (mainly vapour phase contacting with oxygen) technologies to the recovery of chlorine from iron chloride has been the subject of a few patents and literature but these claimed processes have severe limitations that have apparently hindered successful commercial implementation.
Processes based on direct gas phase contacting between iron chloride vapour and oxygen in the absence of a bed of inert material have significant problems due to accretion formation resulting from the production of fine dust of iron oxide.
Beneficiation/partial chlorination in which iron units are added through ilmenite or rutile based slag to control the carbon/chloride ratio in the waste is impractical because control of the waste composition to optimise the dechlorination unit cannot be achieved as indicated in U.S. Pat. No. 4,624,843.
Pre-oxidation of the waste to 500-800xc2x0 C. (U.S. Pat. No. 4,060,584) to apparently convert the ferrous chloride to ferric chloride vapour and iron oxide, the ferric chloride vapour being separated from the iron oxide using high temperature separation and subsequently dechlorinating the iron chloride vapour in a multi stage process will not recover the significant amount of chlorine associated with the other metals which will stay with the solids in the pre-oxidiser.
It is also important to note that the patents ignore the fact that the chlorine associated with iron is only a proportion of the total chlorine available for recovery and the chlorine associated with the other elements such as Mn, Mg and Al and other elements need to be recovered.
We note that carbon and ore blow through occurs routinely in synthetic rutile chlorination plants and recognise that it is difficult to eliminate such blow through and/or implement cost-effective technology for separating the carbon and ore from the chlorides at the exit of the chlorinator. We have also discovered that the major amount of chloride collected in the blown over material is coated on the larger carbon and rutile particles with a minor amount as free chlorides. It is therefore important to recover the chlorine from the chloride coated on the particles and it is not sufficient to remove the carbon and rutile particles by filtration or other means as has been proposed in some of the prior art discussed above.
According to a first aspect of the present invention there is provided a process for recovering chlorine from chlorinator waste including the steps of forming a fluidised bed of chlorinator waste in a fluidising gas containing oxygen and treating the chlorinator waste with oxygen under conditions which promote conversion of metal chlorides into metal oxides and discourage oxidation of carbon contained in the waste.
Preferably the process further includes the steps of separately recovering metal oxides and chlorine gas. Typically the chlorine gas is recycled into the chlorinator in a process for the production of titanium tetrachloride from ilmenite or rutile, including synthetic rutile.
Typically the chlorinator waste contains chlorides such as FeCl2, MnCl2, MgCl2, AlOCl, as well as large quantities of coke and feed material such as synthetic rutile which are blown over from the chlorinator in a process for production of titanium tetrachloride. Accordingly, in a preferred embodiment of the invention the dechlorination of the chloride is achieved by controlling superficial velocity of the fluidising gas in the fluidised bed, the proportion of oxygen in the gas fed to the fluidised bed, the oxygen to chlorinator waste feed ratio and the temperature within the fluidised bed, either separately or in combination, so as to maximize the recovery of chlorine gas and/or minimize the conversion of carbon. Under these conditions the dechlorination process strips chloride absorbed to particulate matter in the feed, such as carbon and rutile particles, and from separate chloride particles in the feed.
Preferably the temperature in the fluidised bed is in the range of 400 to 700xc2x0 C., the superficial velocity of the gas is in the range of 0.2 to 1.0 m/s and the oxygen to chlorinator waste stoichiometric ratio, R, is in, the range 0.2 to 1.2. The stoichiometric ratio, R, is the ratio of oxygen supplied to the stoichiometric oxygen required to completely convert the chlorides and carbon to obtain chlorine and carbon dioxide respectively. Typically the temperature in the fluidised bed reactor, the superficial velocity of the gas and the oxygen to waste molar ratio can be selected depending an the composition and morphological characteristics of the chlorides and other particles, including the particle size of the particles, in the chlorinator waste.
According to a second aspect of the present invention there is provided an apparatus for recovering chlorine from chlorinator waste, which apparatus includes fluidised bed reactor, means for introducing chlorinator waste into the fluidised bed reactor, means for introducing a fluidising gas containing oxygen into the fluidised bed reactor and means for controlling oxygen to chlorinator waste molar feed ratio, superficial velocity of fluidising gas and temperature within the fluidised bed reactor.
The fluidised bed reactor may have a conical bottom entry region for the chlorinator waste feed material and fluidising gas Alternatively, the fluidising gas may be fed into the fluidised bed reactor through a distributor plate. The reactor, is appropriately cooled or heated to allow the extraction or addition of heat to the process depending on the energy requirement of the process, which can be calculated based on the waste composition and the operating parameters.
According to a third aspect of the present invention there is provided a system for recovering chlorine from chlorinator waste including the apparatus described above, and further including means for collecting particulate matter which leaves the fluidised bed reactor and means for quenching any unreacted chlorinated compounds, particulates and the chlorine gas stream which leave the fluidised bed reactor.
Typically the means for collecting particulate matter is a cyclone and the means for quenching unreacted chlorinated compounds is a quencher downstream of the cyclone. In this arrangement solid materials are collected in the cyclone and the resultant gas stream passed through the quencher. The quencher is arranged so that condensable vapors are removed by quenching and the cooled chlorine gas stream that results is conveyed to a baghouse or other appropriate separation device to remove any residual fine dust particles so as to obtain a clean chlorine gas stream. The chlorine gas stream can be directed to a chlorinator in a titanium tetrachloride production process.
Preferably the cyclone is maintained at a temperature above the condensation temperature of any unreacted chlorinated compounds so that an effective separation is achieved between the solids and the unreacted chlorinated compounds. Typically the conditions in the cyclone, including the residence time of the particles and the temperature in the cyclone are chosen so as to avoid oxidation of carbon in the cyclone.
According to a fourth aspect of the present invention there is provided a process for the dechlorination of metal chlorides in a mixture including at least one metal chloride and also including carbon, or carbon-containing materials, and other materials such as, but not limited to, metal oxides, the process including forming a fluidised bed of the mixture in a fluidising gas containing oxygen and converting the mixture under conditions which promote conversion of metal chlorides to metal oxides and discourage oxidation of carbon.
Typically the mixture contains ferrous and/or ferric chloride and chlorides such as those of manganese, magnesium, calcium and aluminium. The mixture may also include metal chlorides such as those of V, Cr, Mb, Zr, Na, Ba, Ce, Sr, Si, Be or Cu.